Refractory ceramics are used in a wide variety of industries and in a wide range of applications, often where high temperature, corrosive environments are involved, such as in the manufacture of glass materials, products and components, where ceramic thermocouple protection sheaths, bubbler tubes, orifice rings, stirrers and other process equipment are employed.
For example, the measurement of the temperature of hot molten glass is performed by a thermocouple shielded in a protecting sheath. Such measurement presents a variety of problems, associated with the temperature involved, the high viscosity and abrasiveness of the molten glass, the chemical reactivity of the glass and the combustion atmosphere in which it is heated. The function of the protecting sheath is to retain the thermocouple in an environment where it is shielded from mechanical and chemical damage. Commonly, the protecting sheath is of alumina. However, while such sheaths have high temperature capability they are relatively brittle. Additionally they suffer attack by molten glass which is often .sufficiently severe to cause rapid failure and total loss of protection for the thermocouple.
Platinum group metal (PGM) and platinum group metal alloys have been used as alternative shielding materials, as too have zirconia grain stabilized (ZGS) versions of the alloys. Maximum structural integrity of these sheaths requires metal thicknesses of 0.5-0.8 mm, so the sheaths are very expensive. Internal support by ceramic tubes has allowed this thickness to be reduced to its present commonly used level of approximately 0.3 mm, but results in sheaths which require mechanical design compromises and which are still too intrinsically expensive.
Apparatus fabricated with a metallic substrate that is coated or clad with platinum group metal provide protection and enable the service life to be increased, but application temperatures are limited by substrate melting point. For example, the use of Ni alloy substrates limits working temperature to below 1300.degree. C. and in most cases below 1200.degree. C. The use of refractory metals can extend working temperatures to as high as 1600.degree. C. but the penalty paid for this is the need to protect all surfaces which might ever see temperatures greater than 400.degree.-600.degree. C. Ceramic substrates offer an alternative vehicle for application particularly in this higher temperature range.
Platinum group metal-clad ceramics have traditionally filled this temperature niche. However, the air gap which is present between the metal cladding and the ceramic reduces the responsiveness of the enclosed thermocouple to temperature changes and the sheaths produced suffer from poor resistance to thermal and mechanical shock. Clad ceramics are two-piece structures and the components have the attributes of metals for the cladding, and ceramics for the substrate.
Traditionally, ceramics directly coated with platinum group metals have failed to produce economically articles with durability suitable for use in high temperature and corrosive environments. Traditional coating processes suffer from stress build-up during deposition and generally provide a limitation for the coating thickness via a particular application route. Such thicknesses are generally insufficient to provide the protection required. Where a coating process has been capable of producing coatings of a satisfactory thickness, other problems have traditionally been manifest,: eg with adhesion, and/or mechanical strength, and/or integrity, and/or porosity.
GB 1242996 (Coming Glass) discloses a method of applying a platinum coating onto ceramics by plasma spraying. It is said that flame spraying is not capable of producing adherent, non-porous coatings. We are not aware that coated ceramics produced according to the process of GB 1242996, or by any other process, were ever successfully commercialized.